The invention relates to a liquid crystal display ("LCD") having both active and passive display areas.
LCD's are well known in the art. Such displays are either of the twisted nematic or dichroic dye type; and in order to provide a white-on-black display, a dichroic display is often used. A dichroic display cell comprises two plates of glass with transparent electrodes in a selected pattern on each glass and a liquid crystal containing a dichroic dye trapped in a cavity therebetween. The molecules of the liquid crystal are normally arranged in a helical pattern which causes the dye molecules to assume a helical pattern and absorb any light passing through the cell. A potential difference established between the electrodes causes the liquid crystal and the dye molecules to align themselves perpendicularly to the cell faces allowing light to pass therethrough.
Unlike light emitting displays, LCD's are passive in that they only modulate light and do not generate it. As a consequence, while LCD's are well suited for daylight reading, they are unusable when there is no ambient light unless backlighting or flooding of the LCD is provided. One method of providing light for LCD's involves the use of a transflector which is positioned behind the display. During daytime use, incident light on the front face of the display propagates through the activated elements of the liquid crystal cell and is reflected by the transflector back through the face as light areas. Those portions of the cell which are not in alignment with energized electrodes block the transmission of light to produce a dark background. During night use, a light source behind the transflector floods the rear of the LCD cell with light; and the energized elements of the display transmit the light again producing a light display on a dark background.
In many applications, LCD's may be used in place of electromechanical-type dial indicators with desirable results. LCD's are power efficient; and since they contain no moving parts, failure rate is drastically reduced and life cycle costs are minimized. LCD's function over a wide temperature range, work well under harsh environmental conditions, and, since the LCD's are not restricted by analog or electromechanical limitations, design flexibility is increased. Accordingly, a variety of parameters such as temperature and pressure, speed, torque, position and RPM may be displayed on LCD indicators. Each parameter measurement may be represented on a straight or arcuate fixed element bar graph-type display in combination with a numerical digital readout. However, each indicator should include indicia markings indicating the parameter which is being measured together with suitable units of measurement such as degrees, pounds per square inch, and the like.
According to present technology, all areas of a white-on-black LCD display which are not energized appear black. In order to provide an illuminated display which gives an indication of the parameter being measured and the units of measurement, the measurement read-out as well as the units of measurement and the parameter identification all require energizing electrodes. This approach requires a different electrode pattern to be formed on the LCD cell for each parameter which is to be displayed. Each of the electrodes requires a conductive trace to the outer periphery of the cell which is connected to the driver circuitry. It is often a problem to find vacant areas on the LCD cell across which the traces may be run without crossing other electrode areas. Although it would be possible to reduce the number of electrodes and traces by printing the parameter identification and units of measurement on the face of the LCD, such a presentation would not be visible at night since light from the transflector will not reach the front face of the LCD in any portion of the cell which is not in alignment with an energized electrode.
There is, accordingly, a need in the art for a LCD in which a common fixed element display such as a bar graph may be used for a variety of LCD indicators and in which parameter identification and units of measurement may be presented and back illuminated without the use of electrodes and traces in the LCD cell.